1. Field of the Invention
The present invention relates to a power circuit comprising a transformer.
2. Description of the Related Art
A power circuit comprising a transformer supplies a power fed to a primary side of the transformer to a secondary side in accordance with the winding ratio of a coil on the primary side to a coil on the secondary side, and supplies a power to a load of an electric apparatus or the like which is connected to the secondary side.
The power circuit is provided with a circuit for detecting a short circuit generated in a coil or another circuit on the secondary side of the transformer if any and stopping the supply of the power to the primary side of the transformer, thereby protecting a power source.
The circuit for protecting a power source has been disclosed in Japanese Laid-Open Patent Publication No. Hei 7-227083, for example. In the protecting circuit disclosed in the publication, a light receiving unit of a photocoupler is provided on the primary side of the transformer and a light emitting unit of the photocoupler is provided on the secondary side of the transformer.
When the short circuit is generated on the secondary side of the transformer, the light emitting unit of the photocoupler emits a light. When the light receiving unit of the photocoupler provided on the primary side of the transformer receives the light emitted from the light emitting unit, the supply of the power to the primary side of the transformer is stopped.
In the case in which the short circuit is generated on the secondary side of the transformer, consequently, the supply of the power can be stopped to prevent a semiconductor device or the like constituting the power circuit from being damaged.
According to the circuit disclosed in the Japanese Laid-Open Patent Publication No. Hei 7-227083, however, the light receiving unit is to be provided on the primary side of the transformer and the light emitting unit is to be provided on the secondary side of the transformer as described above.
Accordingly, a circuit element for protecting the power source is to be provided on both the primary and secondary sides of the transformer and a large space is required so that the size of the power circuit cannot be reduced.